They are $1700 + shipping if you can get one. They are hard to come by.
You need to have the pre- march '07 build, non-welded differential. IMO this is the single most important performance mod for the 335. I want to get as close to my original purchase price -- which I think Autotech will try to keep the price at when they are able to get more. If you get them direct from Birds yourself they are a lot more because of shipping and taxes.

To a very good home. Make sure that you have the non-welded ring gear. And make sure that you have a competent installer -- the only East coast folks I know that I would trust are Koala MS in Cleveland, VAC in Philly and Korman in NC.